Derivative Instruments, Hedging Activities and Fair Value Measurements, Derivative Fair Value Amounts (Details) (USD $) In Millions, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Commodity derivatives [Member]
Derivatives, Fair Value [Line Items]
Asset Derivatives	$ 60.8	$ 61.6
Liability Derivatives	46.9	45.5
Derivatives designated as hedging instruments [Member]
Derivatives, Fair Value [Line Items]
Asset Derivatives	78.1	90.5
Liability Derivatives	41.6	211.3
Derivatives designated as hedging instruments [Member] \| Interest rate derivatives [Member]
Derivatives, Fair Value [Line Items]
Asset Derivatives	35.5	45.2
Liability Derivatives	0	175.4
Derivatives designated as hedging instruments [Member] \| Interest rate derivatives [Member] \| Other current assets [Member]
Derivatives, Fair Value [Line Items]
Asset Derivatives	19.8	19.6
Derivatives designated as hedging instruments [Member] \| Interest rate derivatives [Member] \| Other assets [Member]
Derivatives, Fair Value [Line Items]
Asset Derivatives	15.7	25.6
Derivatives designated as hedging instruments [Member] \| Interest rate derivatives [Member] \| Other current liabilities [Member]
Derivatives, Fair Value [Line Items]
Liability Derivatives	0	175.4
Derivatives designated as hedging instruments [Member] \| Interest rate derivatives [Member] \| Other liabilities [Member]
Derivatives, Fair Value [Line Items]
Liability Derivatives	0	0
Derivatives designated as hedging instruments [Member] \| Commodity derivatives [Member]
Derivatives, Fair Value [Line Items]
Asset Derivatives	42.6	45.3
Liability Derivatives	41.6	35.9
Derivatives designated as hedging instruments [Member] \| Commodity derivatives [Member] \| Other current assets [Member]
Derivatives, Fair Value [Line Items]
Asset Derivatives	42.6	45.3
Derivatives designated as hedging instruments [Member] \| Commodity derivatives [Member] \| Other assets [Member]
Derivatives, Fair Value [Line Items]
Asset Derivatives	0	0
Derivatives designated as hedging instruments [Member] \| Commodity derivatives [Member] \| Other current liabilities [Member]
Derivatives, Fair Value [Line Items]
Liability Derivatives	41.6	35.4
Derivatives designated as hedging instruments [Member] \| Commodity derivatives [Member] \| Other liabilities [Member]
Derivatives, Fair Value [Line Items]
Liability Derivatives	0	0.5
Derivatives not designated as hedging instruments [Member]
Derivatives, Fair Value [Line Items]
Asset Derivatives	18.2	16.3
Liability Derivatives	17.7	26.8
Derivatives not designated as hedging instruments [Member] \| Interest rate derivatives [Member]
Derivatives, Fair Value [Line Items]
Asset Derivatives	0	0
Liability Derivatives	12.4	17.2
Derivatives not designated as hedging instruments [Member] \| Interest rate derivatives [Member] \| Other current assets [Member]
Derivatives, Fair Value [Line Items]
Asset Derivatives	0	0
Derivatives not designated as hedging instruments [Member] \| Interest rate derivatives [Member] \| Other assets [Member]
Derivatives, Fair Value [Line Items]
Asset Derivatives	0	0
Derivatives not designated as hedging instruments [Member] \| Interest rate derivatives [Member] \| Other current liabilities [Member]
Derivatives, Fair Value [Line Items]
Liability Derivatives	12.1	12.2
Derivatives not designated as hedging instruments [Member] \| Interest rate derivatives [Member] \| Other liabilities [Member]
Derivatives, Fair Value [Line Items]
Liability Derivatives	0.3	5.0
Derivatives not designated as hedging instruments [Member] \| Commodity derivatives [Member]
Derivatives, Fair Value [Line Items]
Asset Derivatives	18.2	16.3
Liability Derivatives	5.3	9.6
Derivatives not designated as hedging instruments [Member] \| Commodity derivatives [Member] \| Other current assets [Member]
Derivatives, Fair Value [Line Items]
Asset Derivatives	18.0	15.7
Derivatives not designated as hedging instruments [Member] \| Commodity derivatives [Member] \| Other assets [Member]
Derivatives, Fair Value [Line Items]
Asset Derivatives	0.2	0.6
Derivatives not designated as hedging instruments [Member] \| Commodity derivatives [Member] \| Other current liabilities [Member]
Derivatives, Fair Value [Line Items]
Liability Derivatives	3.9	8.9
Derivatives not designated as hedging instruments [Member] \| Commodity derivatives [Member] \| Other liabilities [Member]
Derivatives, Fair Value [Line Items]
Liability Derivatives	$ 1.4	$ 0.7